1. Field of the Invention
The present invention relates to a voltage regulator, more particularly to a fold-back type overcurrent limiting circuit thereof.
2. Description of the Related Art
A circuit as shown in FIG. 3 has been known as a conventional voltage regulator including a fold-back type overcurrent limiting circuit (for example, see JP 07-074976 B (FIG. 1)).
The voltage regulator section includes a reference voltage source 100, an error amplifier 101, a P-channel enhancement type MOS driver transistor 102, and a voltage dividing circuit composed of resistors 106 and 107. The error amplifier 101 compares a feedback voltage with a reference voltage and adjusts a gate voltage of the P-channel enhancement type MOS driver transistor 102 so that both voltages coincide with each other.
The fold-back type overcurrent limiting circuit is composed of the P-channel enhancement type MOS driver transistor 102, a P-channel enhancement type MOS sense transistor 103 in which the gate and the source thereof are common to the P-channel enhancement type MOS driver transistor 102, a resistor 108, an N-channel enhancement type MOS transistor 105, a resistor 109, and a P-channel enhancement type MOS transistor 104. One end of the resistor 108 is connected with the drain of the P-channel enhancement type MOS sense transistor 103 and the other end thereof is connected with an output voltage terminal 201. The gate of the N-channel enhancement type MOS transistor 105 is connected with the drain of the P-channel enhancement type MOS sense transistor 103, the source thereof is connected with the output voltage terminal 201, and the back gate thereof is grounded. One end of the resistor 109 is connected with the drain of the N-channel enhancement type MOS transistor 105 and the other end thereof is connected with a power source terminal. The gate of the P-channel enhancement type MOS transistor 104 is connected with the drain of the N-channel enhancement type MOS transistor 105, the source thereof is connected with the power source terminal, and the drain thereof is connected with the output voltage terminal of the error amplifier 101, the gate of the P-channel enhancement type MOS sense transistor 103, and the gate of the P-channel enhancement type MOS driver transistor 102.
When an input power source voltage and an output voltage are small in the conventional fold-back type overcurrent limiting circuit, that is, when a difference between input and output voltages is small, the fold-back type overcurrent limiting circuit does not operate. Accordingly, the output voltage does not lower to a level at which the supply of an output current from the P-channel enhancement type MOS driver transistor 102 becomes impossible, so that a relationship between the output voltage and the output current tend to become a relationship as shown in FIG. 4.
In order to make a change for the better regarding this point, a voltage regulator including a drooping type overcurrent limiting circuit in addition to the conventional fold-back type overcurrent limiting circuit has been devised. FIG. 5 shows a circuit example of the voltage regulator. In FIG. 5, the drooping type overcurrent limiting circuit is composed of the P-channel enhancement type MOS driver transistor 102, a P-channel enhancement type MOS sense transistor 110 in which the gate and the source thereof are common to the P-channel enhancement type MOS driver transistor 102, a resistor 111, an N-channel enhancement type MOS transistor 112, a resistor 113, and a P-channel enhancement type MOS transistor 114. One end of the resistor 111 is connected with the drain of the P-channel enhancement type MOS sense transistor 110 and the other end thereof is grounded. The gate of the N-channel enhancement type MOS transistor 112 is connected with the drain of the P-channel enhancement type MOS sense transistor 110, the source thereof is grounded. One end of the resistor 113 is connected with the drain of the N-channel enhancement type MOS transistor 112 and the other end thereof is connected with an input power source terminal. The gate of the P-channel enhancement type MOS transistor 114 is connected with the drain of the N-channel enhancement type MOS transistor 112, the source thereof is connected with the input power source terminal, and the drain thereof is connected with the output voltage terminal of the error amplifier 101, the gate of the P-channel enhancement type MOS sense transistor 110, and the gate of the P-channel enhancement type MOS driver transistor 102.
Even in the case where an input power source voltage and an output voltage are small in the circuit shown in FIG. 5, that is, even in the case where a difference between input and output voltages is small, when an output current becomes larger, first, the drooping type overcurrent limiting circuit operates to limit an overcurrent, thereby reducing the output voltage. Consequently, a difference between the input power source voltage and the output voltage becomes larger. Thus, the fold-back type overcurrent limiting circuit operates, with the result that a relationship between the output voltage and the output current becomes a relationship as shown in FIG. 6.
As described above, according to the conventional voltage regulator including the fold-back type overcurrent limiting circuit as shown in FIG. 3, when the input power source voltage and the output voltage are small, that is, when the difference between the input and output voltages is small, the fold-back type overcurrent limiting circuit does not operate. Accordingly, the output voltage does not lower to a level at which the supply of the output current from the P-channel enhancement type MOS driver transistor 102 becomes impossible, so that the relationship between the output voltage and the output current tend to become the relationship as shown in FIG. 4.
On the other hand, as a circuit for solving such problem, the voltage regulator including both the fold-back type overcurrent limiting circuit and the drooping type overcurrent limiting circuit as shown in FIG. 5 is given. However, because the voltage regulator includes both the fold-back type overcurrent limiting circuit and the drooping type overcurrent limiting circuit, there is a drawback that a circuit scale is increased.